1. Field of the Invention
This invention relates to a semiconductor device for providing electrical overstress protection in particular by shorting to ground excessively high voltage transients.
2. Brief Description of the Prior Art
Semiconductors are becoming widely used in modern telephone systems which has radically altered the need for protection against overvoltage transients caused by, for instance, lightning strikes. At the subscriber line interface in a telephone exchange, protection is conventionally provided by semiconductor secondary protectors which can withstand voltages up to 2 kV. These protectors capture the residual overvoltage stress which has been let through by the primary protectors. On the main distribution function (MDF) of the exchange, extremely high energy dissipation requirements have so far precluded the use of a semiconductor device for primary protection. This requirement is generally met by gas discharge tubes which initially clip the voltage transients at approximately 600-700 V and eventually switch to a low resistance mode. Although these devices are capable of absorbing high energies, their protection characteristics deteriorate with time and more of the overvoltage stress is allowed through to the subscriber line interface.
A semiconductor transient suppressor structure is described in UK Patent No. 2,113,907. It is a thyristor type structure with a controlled holding current and an additional n-layer through which reverse breakdown of the central junction of the structure takes place to control the initial avalanche breakover voltage. The controlled holding current of this type of suppressor may result from the provision of an array of shorting dots of gate electrode material perforating the cathode so as to join the cathode resistively to the gate. In an avalanche structure, the breakdown always starts at localized sites called microplasmas. The localized high voltage gradients set up due to these microplasmas are primarily responsible for the conduction spread during the breakdown period (This effect is different from the more conventional plasma spreading during the post turn on phase.). As the breakdown current increase, the microplasma area increases until a critical current density is reached where the device latches on regeneratively and the voltage collapses to a low value.
In the structure described in the above patent the turn on tends to start in a limited area where the energy dissipation can be very high and conduction spreads only relatively slowly across the whole structure. During this turn on phase the energy dissipation in the limited area can become sufficiently high to cause destruction of the device. This is clearly a disadvantage.